Carbonitriding treatment has been known as measures for improving durability of a bearing component included in a rolling bearing (see, for example, Japanese Patent Laying-Open No. 2007-277648 (PTD 1) and Japanese Patent Laying-Open No. 2008-267402 (PTD 2)). The carbonitriding treatment is heat treatment in which steel is heated to a temperature not lower than an A1 transformation point for penetration and diffusion of carbon and nitrogen into steel and thereafter is quenched. The carbonitriding treatment is effective for extending scratched contact life which is important performance of the rolling bearing.
Other important functions of the rolling bearing include lower torque during rotary motion of a machine, and in order to achieve this, reduction in energy loss during rotation is important. Causes for energy loss during rotation of the rolling bearing include rolling viscous resistance, differential slip, elastic hysteresis loss, or stirring resistance of a lubricating oil. Based on comparison between a ball bearing and a roller bearing in the rolling bearing, energy loss is less in the ball bearing owing to a difference in rolling viscous resistance. Therefore, from a point of view of lower torque during rotary motion of the machine, the ball bearing can be concluded as a more preferred structure.
Since the roller bearing is greater in area of contact between bearing components (rolling bearing rings or balls) than the ball bearing, a contact pressure applied to the bearing component is lower. Therefore, from a point of view of bearing a greater load, the roller bearing is effective. When a great load is born by the ball bearing, however, the contact pressure is excessive and plastic deformation occurs in the rolling bearing ring or the ball. Then, torque in the rotary motion of the machine is higher, generation of unusual noise or lowering in rotation accuracy takes place, and consequently it becomes difficult to fulfill a function as a bearing. Thus, excessive load (contact pressure) which leads to functional failure as a bearing is called a “static load capacity” of the rolling bearing. Therefore, in lowering torque of the rolling bearing, a ball bearing made of a material less likely to plastically deform (high in yield strength) is preferably adopted.
A representative material which is less likely to plastically deform includes a ceramic material such as silicon nitride. This ceramic material, however, is difficult to work while it is less likely to plastically deform. Therefore, when the ceramic material is adopted as a material for a bearing, cost for working disadvantageously increases. Therefore, in place of a ceramic material, development of a material of which total cost including cost for working is low and which is less likely to plastically deform has been demanded. “Less likeliness of plastic deformation” does not relate to a hardness expressed by an HV hardness or an HRC hardness but to a region where an amount of plastic deformation close to elastic limit or yield strength is small.